C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  stl map subclassing          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message prem Guest Posted: Fri Apr 29, 2005 8:41 am    Post subject: stl map subclassing Hi, I'm subclassing the map class as follows .. ---------------- template <typename T1,typename T2> struct ExtendedMap:public map<T1,T2> { ~ExtendedMap() { map<T1,T2>::iterator iter; for(iter = begin() ; iter!=end() ; iter++) { //iter->second.destroy(); } } }; typedef ExtendedMap<uint32_t,uint32_t> testmap; ---------------- at the declaration of map<T1,T2>::iterator iter; the g++ compiler is throwing me this warning. std::map<T1, T2, std::less<_Key>, std::allocator<std::pair _Tp> > >::iterator ' is implicitly a typename But I tried this declaration : iterator iter; as iterator is a public typedef in map . But it did not even compile saying that iterator is not recognized. How do I get around this ??? Thanks. prem [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sumit Rajan Guest Posted: Fri Apr 29, 2005 4:38 pm    Post subject: Re: stl map subclassing "prem" <contactprem (AT) gmail (DOT) com> wrote Quote: Hi, I'm subclassing the map class as follows .. ---------------- template <typename T1,typename T2 struct ExtendedMap:public map { ~ExtendedMap() { map for(iter = begin() ; iter!=end() ; iter++) Apart from the correction I suggested in my earlier post, you may also like to change the above line to: for(iter = this->begin() ; iter!=this->end() ; iter++) See: http://www.parashift.com/c++-faq-lite/templates.html#faq-35.12 Regards, Sumit. -- Sumit Rajan <sumit.rajan (AT) gmail (DOT) com> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sumit Rajan Guest Posted: Fri Apr 29, 2005 4:40 pm    Post subject: Re: stl map subclassing "prem" <contactprem (AT) gmail (DOT) com> wrote Quote: Hi, I'm subclassing the map class as follows .. ---------------- template <typename T1,typename T2 struct ExtendedMap:public map { ~ExtendedMap() { map typename map<T1,T2>::iterator iter; should take care of your immediate problem. Quote: for(iter = begin() ; iter!=end() ; iter++) { //iter->second.destroy(); } } }; typedef ExtendedMap<uint32_t,uint32_t> testmap; ---------------- at the declaration of map<T1,T2>::iterator iter; the g++ compiler is throwing me this warning. std::map<T1, T2, std::less<_Key>, std::allocator<std::pair _Tp> > >::iterator ' is implicitly a typename But I tried this declaration : iterator iter; as iterator is a public typedef in map . But it did not even compile saying that iterator is not recognized. Regards, Sumit. -- Sumit Rajan <sumit.rajan (AT) gmail (DOT) com> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carlos Moreno Guest Posted: Sat Apr 30, 2005 11:20 am    Post subject: Re: stl map subclassing prem wrote: Quote: Hi, I'm subclassing the map class as follows .. ---------------- template <typename T1,typename T2 struct ExtendedMap:public map { ~ExtendedMap() { map for(iter = begin() ; iter!=end() ; iter++) { //iter->second.destroy(); } } }; typedef ExtendedMap<uint32_t,uint32_t> testmap; ---------------- at the declaration of map<T1,T2>::iterator iter; the g++ compiler is throwing me this warning. std::map<T1, T2, std::less<_Key>, std::allocator<std::pair _Tp> > >::iterator ' is implicitly a typename It's asking you to explicitly use the keyword typename in that declaration: typename map<T1,T2>::iterator iter; BTW, what you're doing is wrong... Plain wrong. One should *never* derive from standard library container classes. They're not designed as base classes (e.g., they don't have a virtual destructor), and so deriving from them is asking for trouble. You could simply create your class and put an std::map as one of its data members, and create your member functions that map to std::map's functions as needed. HTH, Carlos -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nickolay Merkin Guest Posted: Sat Apr 30, 2005 11:21 am    Post subject: Re: stl map subclassing Quote: How do I get around this ??? just add 'typename' :) typename map<T1,T2>::iterator it; or reintroduce this typedef typedef typename map<T1,T2>::iterator iterator; The reason why the compiler notifies you is that it cannot be sure that exact instantiation of map<T1,T2> (it may be specialized!) has 'iterator' identifier at all, or that it's a typedef. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nickolay Merkin Guest Posted: Sat Apr 30, 2005 11:22 am    Post subject: Re: stl map subclassing By the way, if you need to clean data of the map entries, maybe it is better to store some smart classes instead of original data? Something like shared_ptr... Because otherwise you should take into account all possible ways to delete entries. - in the map's destructor - in erase() and clear() - probably, in assignment (rewriting old value) The last case is worst, because map::operator[] returns a reference to a value, and it doesn't know if you just read or rewrite it. To eliminate this problem, you could replace it, and return a proxy object with overloaded operator=()... but it is not enough: one can retrieve a T2& and modify it somewhere else. Thus, the best solution is using smart objects such as shared_ptr. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Sat Apr 30, 2005 11:31 am    Post subject: Re: stl map subclassing prem wrote: Quote: Hi, I'm subclassing the map class as follows .. ---------------- template <typename T1,typename T2 struct ExtendedMap:public map { ~ExtendedMap() { map for(iter = begin() ; iter!=end() ; iter++) { //iter->second.destroy(); } } }; typedef ExtendedMap<uint32_t,uint32_t> testmap; ---------------- at the declaration of map<T1,T2>::iterator iter; the g++ compiler is throwing me this warning. std::map<T1, T2, std::less<_Key>, std::allocator<std::pair _Tp> > >::iterator ' is implicitly a typename I wrote an explanation of this at <http://womble.decadentplace.org.uk/c++/template-faq.html#disambiguation>. Quote: But I tried this declaration : iterator iter; as iterator is a public typedef in map . But it did not even compile saying that iterator is not recognized. This behaviour is correct; see <http://womble.decadentplace.org.uk/c++/template-faq.html#base-lookup>. Quote: How do I get around this ??? Add "typename" before the declaration of iter. -- Ben Hutchings Having problems with C++ templates? Your questions may be answered by <http://womble.decadentplace.org.uk/c++/template-faq.html>. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stephen Howe Guest Posted: Wed May 04, 2005 8:12 am    Post subject: Re: stl map subclassing Quote: BTW, what you're doing is wrong... Plain wrong. One should *never* derive from standard library container classes. You mean, one should never publically derive from standard library container classes (because of the point you mentioned). Nothing wrong with protected or private derivation. Stephen Howe [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ Language (Moderated) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.